During the ascent and when entering the atmosphere of celestial bodies, that is, during re-entry into the earth atmosphere as well as during entry into the atmosphere of planets and moons, spacecraft are subjected to extreme aerodynamic, aerothermal, mechanical and acoustic stresses. During this very important flight phase, spacecraft have to be provided with an effective thermal protection which withstands the multiple mechanical, thermal and thermo-mechanical stress situations. In the case of reusable space transport vehicles, such as the U.S. Space Shuttle, the Russian Buran or the future Japanese Hope space glider, ceramic tiles or shingles are provided as a thermal protection at the hot sites, such as the underside, the nose area and the leading edges of the wings.
Future missiles will be moved in a radar-controlled manner at extreme speeds in the direction of their target, the newest developments flying at more than 6 times the speed of sound. As a result of aerodynamic heating, very high temperatures occur at the nose cone of such missiles. During the short flight, suitable thermal protection systems have to prevent heat from penetrating through the nose cone so that the functions of the radar equipment situated behind the nose cone are impaired. Simultaneously, these thermal protection systems (radomes) have to be radar-transparent at the corresponding frequencies in the gigahertz range and have to remain so in all flight conditions.
Dielectric windows (radomes or antenna covers) are used for the protection of antennas on aerodynamic vehicles (missiles) against environmental influences. Radomes have to have a high transparency and a low loss for radar waves in the entire operating frequency range. They should be able to withstand position-caused aerodynamic forces as well as have a sufficiently high resistance to temperatures occurring as a result of aerodynamic heating. Furthermore, they have to be capable of protecting the sensitive antennas accommodated in the radome against heat.
Current radomes, which are built for rockets which have to survive only a short flight (0.5 to 2 min.) at speeds of up to 4 Mach, have to be very thin in order to be able to be used at the corresponding frequencies of less than 18 gigahertz. Such radomes usually consist of a glass-fiber-reinforced polyimide. They may also consist of two thin layers of glass-fiber-reinforced polyimide which are separated by a glass polyimide honeycomb structure (sandwich construction). The disadvantage of such radomes is that the upper limit of the usage temperature of from 650 to 760° C.
Future generations of missiles will be flying significantly faster (over Mach 6) and also significantly longer. This will cast the plastic matrix of the radomes is heated considerably (peak temperatures of clearly higher than 1,000° C.) and pyrolized while forming carbon. This dramatically impairs the radar-optimal characteristics and no longer meets the required transmission characteristics. Such radomes are therefore unsuitable for future missiles.
A conventional radome in a sandwich construction is known from U.S. Patent Document U.S. Pat. No. 5,738,750, which consists of a ceramic core made of a honeycomb structure (SiO2, Al2O3 or AlSiO4 fibers impregnated with polysilicone and/or polysilazane) or of a foam, and consists of two cover layers made of a silica glass fabric impregnated with inorganic resin, the inorganic resin (a polysilicone and/or polysilazane) being partially converted to SiO2 by way of a pyrolysis process. The material is essentially free of elementary carbon. As a result of the incomplete pyrolysis at relatively low temperatures, the formation of elementary carbon is largely prevented during the production. However, this does not mean that the material is free of carbon. It is known that a glass-like SiOxCy phase (black glass) forms during the pyrolysis of inorganic resins, such a polysilicone and polysilazane, in which phase the carbon is embedded. Furthermore, the material described in U.S. Pat. No. 5,738,750 has an upper usage temperature limit in the oxygen-containing atmosphere of approximately 1,090° C. because, near that temperature, elementary carbon separates from the SiOxCy phase, which leads to a clear deterioration of the dielectric characteristics.
Conventional processes for producing filament-reinforced ceramics are essentially divided into gaseous phase processes (CVI) or liquid phase processes (LMI, LPI, SIHP, Sol-Gel). CVI processes have the disadvantage that they are very cost-intensive and difficult to control. Virtually only liquid phase processes are therefore used for the production of filament-reinforced ceramics.
In the case of the LMI process (Liquid Melt Infiltration), the building-up of the matrix takes place by the infiltration of a fiber preform by means of molten metal and a simultaneous or subsequent oxidation. Here, the disadvantages are the difficult process control, the fiber corrosion as a result of the molten metal and the retention of residual metal as a result of incomplete oxidation.
In the case of the LPI process (Liquid Polymer Infiltration), the building-up of the matrix takes place by infiltrating the semifinished fiber products with suspensions which contain an inorganic polymer which, in a pyrolysis, can be preferentially converted to covalently bound, amorphous or crystalline ceramics. The conversion to ceramics is connected with large volume shrinkages and resulting crack formations within the matrix. As a remedy, passive or active fillers are used which, as a result of a volume expansion before or during the pyrolysis, partially counteract the shrinkage. A filling of the crack network and an increase of the matrix density normally takes place in multiple reinfiltration steps which represent considerable time and cost expenditures. As a result of the use of inorganic polymers (precursors), the production of oxidic fiber-reinforced ceramics can take place only to a limited extent because the C-atoms of the precursor are either bound into the diverted amorphous or crystalline structure of the ceramics, or are additionally present as elementary carbon separations. Although the C-fraction can be lowered by means of high-temperature aging under oxidative conditions, the C-fraction can be lowered, this is connected with considerable time and cost expenditures. Another possibility consists of the incomplete pyrolysis of the precursor at low temperatures with the goal of forming as little elementary carbon as possible. However, when such a material is used at high temperatures, the material has an upper time and temperature limit, as elementary carbon forms by further pyrolysis. The production of purely oxidic fiber-reinforced ceramics from inorganic polymers according to the present state of the art is always connected with C impurities. This is a significant disadvantage for a use as a radome material because even the smallest quantities of carbon impair the dielectric characteristics.
In the case of the SIHP process (Slurry Impregnation and Hot Pressing), the building-up of the matrix takes place by infiltration of semifinished fiber products with a suspension at an aqueous or organic base which contains ceramic powder, an organic binder and additional auxiliary agents. The consolidation takes place by hot-pressing or high-temperature isostatic pressing, which conventionally require considerable system-related expenditures and are limited to components having a simple geometry.
In the case of the sol-gel process, the infiltration of semifinished fiber products takes place by means of molecular-disperse or colloid-disperse sols. The transition from the low-viscosity sol to a high-viscosity sol takes place by destabilization or by hydrolysis and polycondensation reactions. By means of this technique, chemically pure oxides can be produced. However, the drying and sintering of gels is connected with very large volume shrinkages, which leads to high porosity of the matrix and to crack formation. Filling in the cracks and reducing the porosity requires multiple reinfiltrations and sintering cycles which are connected with considerable time and cost expenditures.
In the case of the EFD process (Electrophoretic Filtration Deposition), the infiltration of semifinished fiber products takes place by means of colloid-disperse sols. The transition to the gel takes place by electrophoretic deposition on an electrode, the fabric to be infiltrated being placed directly in front of the latter. Although chemically pure oxides can be produced, the high drying shrinkage frequently leads to crack formation. The production of laminates and a further densification of the matrix takes place in an additional process step by means of filter pressing, which may result in density gradients in the laminates and thus limits the production of complicated component geometries.
From U.S. Patent Document U.S. Pat. No. 5,856,252, a method is known in which the infiltration of mutually stacked fabric layers takes place by filter pressing. An aqueous suspension is used which contains a fine-particle oxide ceramic solid. The sintering takes place without pressure, whereby a purely oxidic porous matrix is created which subsequently is reinforced in several precursor reinfiltration steps and sintering cycles. However, this process requires multiple reinfiltration cycles, which result in considerable time and cost expenditures. Additionally, the process technique of filter pressing, which lead to density gradients in the laminate and limits the production of complicated component geometries.
Another process (WHIPOX) is known, in which fiber bundles are infiltrated by means of an aqueous suspension containing mullite preliminary-stage powder, a temporary binder and additional organic auxiliary agents. By means of a winding process, the fiber bundles are deposited as rotational bodies or prepregs and, in the wet state, are further processed to form laminates. After the thermal unbinding and pressureless sintering, a purely oxidic matrix is formed which has a high porosity (60 to 80%). Disadvantages of this process are caused by the winding technique and the high matrix porosity. Laminates can exclusively be built up which are constructed of unidirectionally reinforced layers. Together with the high matrix porosity, this leads to very low off-axis strengths, such as interlaminar shearing strength, intralaminar shearing strength or transversal tensile strength. Because of the winding technique, these characteristics cannot be improved by using three-dimensional reinforcing architectures. Another disadvantage is the use of a temporary binder and of organic auxiliary agents which require an additional process step for the thermal unbinding.
For the production of monolithic ceramic green bodies, essentially five shaping processes are known; specifically, pressing, isopressing, extrusion, injection molding and slip casting. These processes have no significance for the production of filament-reinforced ceramics because they do not meet the requirements with respect to a fiber-preserving complete and homogeneous infiltration of the spaces between the fibers by means of matrix, and also do not permit an economical production of large-surface, light-weight structures having a complicated geometry. Only suspensions as used during slip casting would in principle be suitable for building up the matrix. The consolidation of the slip takes place by withdrawal of the aqueous suspending agent via use of porous plaster molds. This consolidation mechanism by the withdrawal of water has the important disadvantage that a migration of fine particles and soluble constituents will occur which leads to inhomogeneous textures with density gradients. The plaster molds also have to be dried after each use, which represents additional expenditures.
The known disadvantages of slip casting are partially overcome by the process (DCC) known from Swiss Patent Document CH 686 879 A5. In this process, an aqueous suspension with a high content of solids, after decanting into a non-porous mold, is consolidated by changing the surface charge of the particles. The coagulation by changing the surface charge condition is normally achieved by substrate/enzyme reactions which shift the pH-value in the direction of the isoelectric point or increase the ion concentration. The process has the disadvantage of using enzymes which are expensive and often require special storage. They may be inhibited in their effectiveness by interactions with constituents of the slip, which makes the development of the suspension more difficult. Since the solidity of wet green bodies generally rises superproportionally with the content of solids, suspensions are used which have contents of solids between 55 and 60 percent by volume. Sufficient green strengths for removal from the mold and further handling in the wet state are achieved only by using these high contents of solids in the suspension. With respect to possible use for producing filament-reinforced ceramics, the very high solids contents of the suspensions are problematic because, as a result of the connected high viscosities, a homogeneous and complete infiltration of the space between the fibers cannot be achieved. The relatively short consolidation times in the range of between 30 minutes and 2 hours have the result that the viscosity of the suspensions rises considerably after as little as a few minutes, which is much too short as a processing period for conventional infiltrating and laminating techniques.
Another process (HAS), which also partially overcomes the known disadvantages of the slip casting, is known from European Patent Document EP 0 813 508 B1. An aqueous suspension with a high content of solids is also used in this process, to which suspension a metal nitride powder is added for the purpose of consolidation. Non-porous molds are also used. The consolidation takes place by heating the suspension above the hydrolysis temperature of the metal nitride which, when aluminum nitride is used, is at temperatures between 50° C. and 70° C. By means of the hydrolysis, the suspending agent is partially withdrawn from the suspension and, in certain cases, the pH-value is shifted, which leads to a steep increase of the viscosity. One disadvantage of the process is the necessary heating of the suspension and the connected use of heated molds. Since temperature profiles occur in the suspension, an inhomogeneous consolidation and related density gradients are expected. As in the case of the previously mentioned process, the use of very high contents of solids between 50 and 60% by volume is required in the suspension in order to be able to ensure sufficient green strengths for removal from the mold and further handling in the wet state. Because of the high contents of solids, the same problems occur with respect to possible use in the production of filament-reinforced ceramics, since the resulting high viscosities make a homogeneous and complete infiltration of the spaces between the fibers more difficult. Furthermore, the consolidation times are still shorter than in the previously mentioned process, which is much too short as a processing time period for customary infiltrating and laminating techniques.